Hydrobiologia 496: 269–278, 2003. E. Sigvaldad´ ottir, A.S.Y. Mackie, G.V. Helgason, D.J. Reish, J. Svavarsson, S.A. Steingr´ ımsson & G. Guðmundsson (eds), Advances in Polychaete Research. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 269 Comparative study of the diaphragm (gular membrane) in Terebelliformia (Polychaeta, Annelida) Anna E. Zhadan & Alexander B. Tzetlin Department of Invertebrate Zoology, Biological Faculty, Moscow State University, Moscow, 119992, Russia Fax: (7095) 939-4309. E-mail: azhadan@mail.ru; tzetlin@soil.msu.ru Key words: diaphragm, coelom, anatomy, ultrastructure, Terebelliformia Abstract The structure and location of the diaphragm (gular membrane) was studied in five families of Terebelliformia: Terebellidae, Trichobranchidae, Pectinariidae, Ampharetidae and Alvinellidae, using dissections, histology, and scanning and transmission electron microscopy. Position, shape, and structure of the diaphragm differ in these taxa. In Terebellidae and Pectinariidae the diaphragm is straight. In Trichobranchidae, Ampharetidae and Alvinellidae it is funnel-shaped. Diaphragm possesses two contractile sacs in Terebellidae and Pectinariidae, one in Alvinellidae and none at all in Trichobranchidae. The relative size and form of the sacs varied. Representatives of the family Ampharetidae have one or two sacs or none at all. Four kinds of the diaphragm can be distinguished: strait with two sacs, funnel-shaped with two sacs, funnel-shaped with one sac, funnel-shaped without sacs. In some Alvinellidae, the diaphragm is fenestrated, while in all other taxa it is continuous. The wall of the sacs is more muscular than the wall of the remaining diaphragm. The diaphragm is attached to the body wall at different levels: between the third and fourth segments in pectinariids or between the fourth and fifth in terebellids, ampharetids, alvinellids and trichobranchids. In most cases, the diaphragm contains two coelothelial layers with a well-developed extra- cellular matrix in between, and one or two muscle layers. The maximum development of the muscle fibres occurs in Terebellidae; probably related to the length of buccal tentacles. Significance of morphological and ultrastructural peculiarities of the diaphragm is discussed. Introduction The diaphragm (gular membrane) is one of the im- portant characteristics of internal anatomy in Tere- belliformia. It is an anterior septum that distinctly differs from other septa. The presence of a diaphragm in Polychaeta is often connected with such eversible structures as a pharynx, palps, and tentacles (Fauchald & Rouse, 1997). In addition, the diaphragm divides the anterior coelom where funnels of excretory neph- ridia open, from posterior one where gametes mature and funnels of gonoducts are located (Smith, 1992). Although the diaphragm has been found in all ter- ebelliform polychaetes (Hessle, 1917; Fauchald & Rouse, 1997), its structure has been poorly studied. According to Meyer (1887) and Hessle (1917), in Am- pharetidae and Terebellidae the diaphragm attaches to the body wall between the fourth and the fifth segments and in Pectinariidae between the third and the fourth ones. In Terebellidae, the diaphragm is a straight partition with two blind muscular expansions. They contract and pump coelomic fluid in the cavity of the buccal tentacles, thus providing for their exten- sion (Meyer, 1887). In Melinna sp. (Ampharetidae) only one such expansion was observed (Hessle, 1917). Hessle (1917) supposed that the relatively short buccal tentacles of ampharetids and pectinariids extend due to contraction of the whole diaphragm as he did not observe the muscular expansions. In many Terebelli- formia, especially in Terebellides (Trichobranchidae), diaphragm is funnel-shaped. According to Heimler (1983), the diaphragm appears in aulophora larvae of Lanice conchilega (Pallas) between the third and the fourth chaetigers simultaneously with the first tentacle. He supposed that prediaphragmal section was represented by the larval segments.